The "text box" is actually a "text layer". The dashed line around shows the "layer boundary" (outside of which you cannot paint, as long as this layer is the active layer.
If you want to draw a line in the image, you normally add a layer (+ icon at the bottom of the Layers list, or Layer ➤ New layer) and draw on that layer. Make sure when you create the layer that it has the size of the image.
You normally use the Layer list to select the layer on which you want to work (which is one reason why the Layers list is important).
Layers are fundamental in Gimp (and in other image editors). When things are on their own layer, they are easy to move around, paint in a different color, hide, duplicate... Using one single layer makes it very difficult to change your mind without undoing everything back to the point where you want something different.
You should also avoid painting on text layers, if you alter a text layer in any way that doesn't use the text tool, the layer is converted to a plain bitmap layer and you can no longer edit the text.
If you want to draw a line in the image, you normally add a layer (+ icon at the bottom of the Layers list, or Layer ➤ New layer) and draw on that layer. Make sure when you create the layer that it has the size of the image.
You normally use the Layer list to select the layer on which you want to work (which is one reason why the Layers list is important).
Layers are fundamental in Gimp (and in other image editors). When things are on their own layer, they are easy to move around, paint in a different color, hide, duplicate... Using one single layer makes it very difficult to change your mind without undoing everything back to the point where you want something different.
You should also avoid painting on text layers, if you alter a text layer in any way that doesn't use the text tool, the layer is converted to a plain bitmap layer and you can no longer edit the text.